The present invention relates to an image processing apparatus for processing a color image read from an original, for example, in an image forming apparatus such as a digital color copying machine for forming a copy image of a color image, and also to an image forming apparatus such as a digital color copying machine using this image processing apparatus.
Generally speaking, there are various types of originals to be copied, and it is not necessarily ideal to reproduce an image of an original with high fidelity, depending on the purpose of use of the reproduced image. For example, in the case of an original such as newspaper or a notebook, on which the density of a background image is high, if the background image with high density is reproduced with high fidelity, the contrast of character image portions lowers and the copied image becomes very difficult to read.
Besides, where there are images on both sides of a thin paper sheet such as a paper sheet of a magazine, the image on the back side of the sheet is seen through. In such a case, if the image on the front side is copied, the image on the back side is copied together, and a phenomenon called “back-side copying” occurs.
Accordingly, where newspaper, a notebook or a magazine is to be copied, the density of copying is manually adjusted in normal cases so as to reduce the background image or the possibility of back-side copying.
Jpn. Pat. Appln. KOKAI Publication No. 2000-83166, for instance, discloses that a histogram is produced from input image data to calculate white and black reference values, and the density of the input image data is adjusted based on the white and black reference values, whereby the background image is eliminated or the density of thin character images is increased.
In addition, Jpn. Pat. Appln. KOKAI Publication No. 11-187266 discloses that a background density level is calculated from input R-, G- and B-image data, and a density conversion table is prepared based on the background density level, whereby the density of the input image data is converted. Thereby, the background density adjustment, including elimination of background, is performed so that character images may not become difficult to read even if the background density is decreased.
In the case of color copying, however, a background image on an original, if it has a special significance (e.g. a map), should be left as such. In the case of such a colored background image, it is very difficult to determine whether the background image should be left or not.
In the above-mentioned Jpn. Pat. Appln. KOKAI Publication No. 11-187266, the density conversion table is prepared from the R-, G- and B-image data to adjust the density. In normal cases, in an image forming apparatus such as a digital color copying machine, color-separated images of respective color components, which are obtained by a conventional subtractive color mixing method, i.e. four color image signals of yellow (Y), magenta (M), cyan (C) and black (K), are used for image formation. In the case of color copying, high-quality ground processing cannot be performed on the basis of R-, G- and B-image data.
In the case of an original including a photograph, if elimination of background or back-side copying is effected for the entire image, the density of the photograph, which requires high-fidelity density reproduction, lowers. It is necessary, in particular, to prevent the photograph area from being subjected to the process for elimination of background or back-side copying. It is thus very difficult to determine whether the background processing should be carried out or not, depending on the type of an original, e.g. depending on whether an original includes a character or a photograph.